1. Field of the Invention
The present invention relates to a system to measure the condition of a vehicle brake and, more specifically, to a system to detect and generate an electrical signal when the brake actuator travel exceeds a predetermined limit thereby indicating the need for brake service.
2. Description of the Prior Art
The mechanical adjustment of air brakes on trucks and other highway vehicles is a constant source of concern to vehicle owners, drivers and public officials whose duty it is to inspect and to check the road worthiness of such vehicles. In the case of air brakes, various devices have been designed to assist and to expedite the process; some of which are very simple, others of which tend to be relatively complex and, in all likelihood, relatively costly.
A common type of air brake system for trucks utilizes an actuator associated with each brake. The actuator has an opening in one side through which an actuator rod extends, the actuator rod being in turn connected to the actual vehicle brake through a suitable mechanical linkage such as an automatic slack adjuster which is well known in the art. When the vehicle brakes are not being applied, a spring or the like within the actuator holds the push rod in a relatively retracted position when the brake is off. However, when an operator applies the brakes, pressurized air is supplied to the actuator and the pressurized air causes the actuator rod to move along its longitudinal axis, in a direction out of the actuator. This movement continues until the brake engages and provides a return force that balances the force provided by the air pressure.
As the brake wears, the push rod must move through a larger and larger distance from its retracted position to an extended position in which the brake is applied. An automatic slack adjuster takes care of a certain amount of linkage adjustments due to wear but if the process continues uncorrected, the actuator rod travel will gradually reach its upper limit, determined by the geometry of the actuator. When this occurs, the brakes will not be effective even when fully applied by the vehicle operator. Because of this, in the U.S., trucking regulations place an upper limit of two inches on the maximum amount of actuator rod travel between its retracted and extended positions.
Two basic types of solutions have been provided to address the problem of wear in air brake systems. The first of these solutions is a device known as a slack adjuster that is included in the linkage between the actuator rod and the brake. A slack adjuster removes slack from this linkage, thereby reducing the amount of actuator rod travel necessary before the brakes are applied. Both automatic and manual slack adjusters are well known and in use. In an automatic slack adjuster, the slack removal operation is automatic, and occurs whenever a predetermined degree of slack develops in the linkage. However, an automatic slack adjuster is a relatively expensive and complex device, and is subject to occasional malfunction. A manual slack adjuster must be adjusted manually whenever excessive actuator rod travel develops in the brake system. However in general, an operator has no means of readily determining when such wear exists.
In order to overcome the limitations of existing slack adjusters or for use with manual adjustment systems, a variety of brake wear indicators have been devised to provide an indication to an operator that excess actuator rod travel has developed. For example, U.S. Pat. No. 4,279,214 describes a brake wear indicator that comprises a sleeve that extends around the actuator rod and through the opening in the actuator through which the actuator rod extends. A relatively loose fit is provided between the indicator and the actuator rod, and a comparatively snug fit is provided between the indicator and the actuator. When the brakes are applied, the indicator is forced out of the actuator along with the actuator rod. When the brakes are subsequently released, the fit between the indicator and the actuator prevents the indicator from retracting. Thus at any given time, visual inspection of the indicator will indicate the maximum travel of the actuator rod since the indicator was last reset into its retracted position. While effective in principle, the brake wear indicator disclosed in this patent suffers from the problem that there is considerable variation in the size of the opening in the side of the actuator through which the actuator rod extends. Thus, in practice, indicators of various outside diameters must be provided in order to cover the different brands and models of actuators currently in use. In addition, no electronic signal is generated that can be used to alert the operation or the need for service without a visual inspection of each brake assembly.
One of the simplest kind of devices is a split ring plastic sleeve designed to slip over the air pressure chamber actuator rod, and thereafter provide a visual indication of actuator rod travel distance or stroke extension. Excessive actuator rod travel indicates a level of brake lining wear that warrants replacement of the linings. Another device which likewise acts as a visual indicator involves modification to the actuator rod itself. A colored band is painted into a groove circumferentially machined in the actuator rod.
U.S. Pat. No. 3,298,466 discloses a brake wear indicator that comprises an extension of the return spring of the actuating cylinder that extends from the interior of the actuating cylinder to the exterior where it is visible for inspection. When the brakes are applied, the return spring is compressed and the extension projects from the actuating cylinder. The greater the distance the extension projects the greater the wear of the brake linings.
The foregoing and other visual indicators are disadvantageous by reason of their dependence on vision. The ability to see can be hampered by darkness, or by dirt, mud or slush picked up form the road. Further, it can be an awkward and sometimes messy job to move into a position that enables a visual check to be made. Also, more importantly, no warning is given to the operator in the cab so that immediate service can be obtained.
Prior art slack adjusters have used switching devices to signal when the brake adjuster has reached the end of its available travel thereby signaling the need for brake service. Examples of such devices are disclosed in U.S. Pat. Nos. 3,776,329; 5,253,735; 5,285,190; 5,339,069 and 5,358,075 the disclosures of which are hereby incorporated by reference. The switch, which is mounted to the slack adjuster, is electrically connected to a monitoring device using two connecting wires. These types of devices function to electrically signal when the brake slack adjuster has traveled to its service limit thereby notifying the operator of the need for brake service.
Another method of signaling when brake service is required is a system to monitor the travel of the pushrod of the brake actuator. One such device is described in U.S. Pat. No. 4,757,300 to Sebalos, the disclosure of which is hereby incorporated by reference. According to Sebalos, a magnet is secured to the actuator rod of a brake actuator which magnetically activates a switch mounted to the housing of the brake actuator when the rod travels to its service limit. The switch is connected to an electronic circuit to alert the operator of the need for brake service.
U.S. Pat. No. 5,310,023, the disclosure of which is hereby incorporated by reference, describes a sensor for detection of the overtravel of a brake actuator rod having a piston using a pair of conductors attached to an actuator piston housing such that an electrical circuit is closed when the piston travels to the section of the actuator piston housing incorporating the conductors which represents a travel limit condition thereby completing the circuit.
U.S. Pat. No. 5,450,930, the disclosure of which is hereby incorporated by reference, uses a sensor mounted inside the actuator housing where the sensor is activated by a structure attached to the actuation rod. The sensor can be a magnetic structure, a contacting switch(es) or a load cell.
These and other prior art systems have the limitations of overly complicated structures that must be supported and are prone to failure. In the case of visual systems, no signal is directly available to the driver while operating the vehicle which could result in an overworn brake condition prior to a service inspection.